Brown is one of the few major economy leaders who has announced plans to attend the UN-led 7-18 December conference in Copenhagen.

The meeting is intended for environment ministers, and meant to sign a new deal to extend or replace the existing Kyoto Protocol after 2012.

“I believe that leaders must engage directly to break the impasse," Brown told energy and environment ministers and representatives of 17 of the world’s main polluting nations, gathered in London.

“I urge my fellow leaders to work together to reach agreement amongst us," he said.

Many analysts and lawmakers doubt the world can agree a deal in December, arguing the deadline is too tight given a lack of progress on issues including emissions reduction targets.

“I believe agreement at Copenhagen is possible," Brown said, adding: “But we must frankly face the plain fact that our negotiators are not getting to agreement quickly enough."

The two-year UN talks launched in Bali, Indonesia in 2007 are split on how big carbon cuts rich nations should make by 2020, and how much they should pay developing countries to prepare for and slow global warming.

The 18-19 October London meeting is the latest in a US-instigated series of major economies forums (MEF) meant to build support for the UN process.

“The London talks focused on how to turn a patchwork of national policies into an international deal, as well as on climate finance and technology cooperation," said Todd Stern, Washington’s top climate envoy, on Sunday.

Stern could not confirm that the US would bring either a concrete emissions reduction target or a dollar offer to Copenhagen.

As instigators of the MEF series, the US is co-chair of the London meeting alongside the host country Britain. Scientists have issued increasingly stark warnings of the possible climate threats facing the planet, for example from melting Arctic ice sheets which could stoke further warming.

“Success at Copenhagen is still within reach. But if we falter, the earth itself will be at risk," Brown said.